1. Field of the Invention
The present invention relates to a developing device to be employed in an image formation device at which an electrophotography system is applied, such as a copier, a printer, a fax machine, a multifunction device including functions thereof and to an image formation device which employs this developing device.
2. Description of the Related Art
Conventionally, a developing device which is charged with a developing agent (a “two-component developer” formed of a toner and a magnetic carrier) is mounted at an image formation device at which an electrophotography system is applied, such as a copier, a printer, a fax machine, a multifunction device including functions thereof, or the like. This developing device (a developing roll) is disposed to oppose a photosensitive body, the toner is adhered to a surface of the photosensitive body, and an electrostatic latent image is made visible with the toner. Thus, image formation is implemented.
In order to improve movement of the magnetic carrier within a development nipping area and raise developing efficiency of such a two-component developer, lowering of carrier magnetizations has been implemented (see Japanese Patent Application Laid-Open (JP-A) No. 62-38476, JP-A No. 11-27484 and suchlike).
However, when magnetic flux density of a developing magnetic pole is lowered and/or carrier magnetization is lowered, there is a problem in that adherence of carrier to an image-bearing body is more likely to occur, particularly in conditions of high temperature and high humidity, low temperature and low humidity, and the like.
In particular, because a toner with small particle diameters has a larger overall toner surface area, in order to use such a toner, it is desirable to reduce particle diameters of the carrier and increase surface area of the carrier. However, in such a case, magnetic attraction of the carrier is lowered, and therefore, adherence of the carrier to an image-bearing body is more likely to occur.
Adherence of carrier to an image-bearing body is likely to occur when a contrast between respective potentials at an image portion and a non-image portion is large. When carrier adherence occurs at an image portion, because the carrier particles are larger than the toner particles, a toner image and a transfer object body cannot be tightly contacted in the vicinity of an adhered carrier particle, and transfer dropouts will occur at and around these carrier particles.
Furthermore, if carrier adherence occurs at either of a non-image portion and an image portion, carrier particles that are not transferred can cause damage to a cleaner, the photosensitive body and the like, while carrier particles that are transferred are developed in the form of stains in an image, and can cause damage to a fixing device.